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OnlineThe goal of this tutorial is to clarify issues that are relevant to a
discussion about light booths. The following topics will be discussed:
What is a Lightbooth
Ways to Quantify Daylight
Correlated Color Temperature
Spectral Power Distribution (SPD)
Grading the Quality of Daylight Reproduction
Technical Difficulties in Producing Accurate Artificial Daylight
Influence of the Automotive Industry.
Calibrating a Light Booth: Improves Quality Control.
Products that provide calibrated Daylight.
Light booths are a technical device designed to provide a standard set of
lighting conditions for visual color inspection. The three most common types
of light sources in the world are incandescent, fluorescent, and daylight.
Two other light sources are sometimes found in light booths. Ultraviolet
(UV) light, which is used for observing fluorescence in various materials and
"horizon" light which meant to approximate the light of a setting sun.
These various lighting conditions provide the user with the capability of
comparing the color of their product to a master from their customer.
Another use for light booths is in comparing one run of a material with
another. In theory, if the product's color matches under the various lights
in your booth, it will match in real world conditions. Unfortunately, in the
real world there are some important issues involving the calibration and
consistency of light booths that have only recently begun to be addressed.
Typically, "daylight" is the most important light that a light booth provides
because it is the light by which the most critical color checking is done. A
problem in the light booth industry is that different manufacturers have
different simulations of daylight and almost all light booths do not provide
the user with the ability to quickly and easily calibrate the booth's
"daylight" to a recognized standard. Tailored Lighting has developed a
solution to the calibration problem which will be discussed later.
Calibration involves making quantitative measurements and doing comparisons
to agreed upon standards. There are two ways of quantifying daylight and ways
of grading how well the artificial daylight compares to various standards.
The following section explains the two most commonly used ways to quantify
daylight.
When someone says "daylight," what do they mean? There is daylight during a
sunny day, a cloudy day, in the shade, and at different times of the day and
during different seasons and at different latitudes on the earth. All of
these types of daylight can have different characteristics which can cause a
change in an object's color appearance. Fortunately, the differences in
these types of daylight can be quantified.
The two properties of daylight that need to be quantified are: how intense it
is and what "color" (or tone) it is. The later quantity, color or tone, is
the most important quantity that needs to be calibrated because of the strong
affect it has on the apparent color of an object. The intensity of the
light inside a light booth is of less importance provided that it exceeds
about 80 foot-candles and is less than 150 foot-candles. Since the tone of the
daylight is so important, the rest of this section will focus on the
different methods of quantifying it.
There are two frequently used methods of quantifying the "tone" of daylight
and it is important to know when it is acceptable to use the simpler of the
two and when it is not acceptable. The simplest and most common method of
quantifying different types of daylight is with the metric correlated color
temperature. The more complicated (and more accurate) method is with a
quantity typically referred to as a Spectral Power Distribution, or SPD.
Relax, these two quantities are not that difficult to understand.
Correlated Color Temperature (or more simply Color Temperature (CT)) is a
metric that is used to quantify the tone of the light being used for viewing.
The easiest way to understand color temperature is to think about heating up
a nail. The first color one observes being emitted from the nail as it
climbs in temperature, is a deep red. As more energy is added to the nail,
its temperature increases and the light it emits changes from red to orange
to yellow to white, and eventually to shades of blue. At different
temperatures, the color of the light coming from the nail is different and we
can correlate the color of the emitted light with the temperature of the
nail. Hence the term "correlated color temperature." The unit of measure
for color temperature is Kelvin (K). (Actually, a typical nail would melt
before it turned white and then blue, but for this discussion, the nail is
made of "Unobtanium" a yet to be discovered metal that never melts.)
REMEMBER! Color temperature is associated with the apparent color of the
light emitted by the nail and NOT the intensity of the light being emitted.
Color temperature and light intensity are two independent quantities. The
following table of frequently encountered lighting conditions and the
approximate color temperature of each may prove useful:
Setting sun ~2300 K
Typical light bulb 2800 - 2900 K
Typical Tungsten-Halogen bulb 2900 - 3100 K
Direct sunlight (no sky) ~5000 K
Direct sunlight with blue sky ~5500 K
Cloudy overcast day 6500 - 8000 K
Clear blue north sky (with no direct sunlight) 7500 K and higher
Table of common lighting conditions and their approximate color temperature
Note that color temperature runs opposite of how a designer would think of
light. For designers, a light with a lot of yellow and red (ex.
incandescent) would be referred to as "warm" and a light that is very white
or blue would be referred to as "cool." For example, a designer would say
that light from a setting sun is "warm" and daylight during a cloudy day is
"cool." However, the color temperature of light on a cloudy day is higher
than that from a setting sun since it is more blue in tone.
It is important to understand the concept of color temperature because the
governing body of color, the CIE, refers to three main color temperatures
when discussing daylight. They are referred to as D55, D65, & D75. The "D"
is shorthand for "daylight" and the 55, 65, and 75 refer to 5500K, 6500K, and
7500K respectively.
Spectral Power Distribution (SPD)
The SPD is a more accurate way of quantifying daylight than color temperature
because it measures the amount of power (watts) at each wavelength (typically
in 5 or 10 nm steps) in the visible spectrum. The advantage of SPD is that
it gives you a more complete picture of the illuminating source. The SPD's
main disadvantage is that the instrument required for its measurement, a
calibrated spectroradiometer, is very expensive (usually more than $10,000).
Color temperature or SPD: When to use one or the other.
The advantage of using color temperature and intensity to quantify daylight
is that only two independent metrics are needed and the instruments needed to
measure color temperature and intensity are much less expensive and easier to
use than a spectroradiometer. The question is, when can you use color
temperature to characterize daylight and when should you use the SPD.
As a good rule-of-thumb, if the SPD of an artificial daylight source is
smooth, continuous, and does not have sharp and narrow spikes in it, then
color temperature is usually a valid method of characterizing it. The SPD of
actual daylight is relatively smooth and continuous. There are bumps and
peaks in actual daylight, but these variations are relatively small. The SPD
of daylight produced by a filtered incandescent source is typically smooth
and continuous and therefore typically lends itself to color temperature
measurements for calibration purposes. Daylight fluorescent sources are
another story.
When compared to actual daylight, "daylight" fluorescent sources are not
smooth and continuous and they have sharp and narrow spikes in their spectrum
that can be an order of magnitude or more higher than a near-by point in the
SPD. Because of this, users should always look at a high resolution SPD of a
"daylight" fluorescent source when making a comparison with it and a standard
or actual daylight.
Grading the Quality of Daylight Reproduction.
The most official method of grading the quality of daylight simulation was
designed by the CIE. That method is #51-A Method for Assessing the Quality
of Daylight Simulators for Colorimetry which is referred to by the ASTM
Committee on Visual Methods: D 1729-89 Standard Practice for Visual
Evaluation of Color Differences of Opaque Materials.
CIE method #51 is a fairly complicated calculation that grades daylight
simulation in two areas: the visible range and the ultraviolet range (for
fluorescent materials). The grading scale for both ranges runs from "A" to
"E" with "A" being the best and "E being the worst. ASTM D1729 requires that
daylight simulators have a grade of "B" in the visible range and "C" in the
ultraviolet range. ColorView gets a "A" grade.
The CIE #51 was published in 1981 and since that time there have been
suggestions on better methods to asses the quality of daylight simulators
(see publications by Heinz Terstiege from BAM).
A more commonly known method for grading daylight is called the CRI which
stands for Color Rendering Index. The range of the CRI is from 0 to 100 with
100 being the best. In theory, the CRI is suppose to compare the color
rendering ability of an actual light source operating at a given color
temperature to that of a "black body" at a the same color temperature.
The CRI is most frequently used by daylight fluorescent manufacturers to try
and convince people that the light provided by their bulb has good color
rendering abilities. However, in practice, the CRI has its limitations. The
calculation of CRI relies on "the use of a specific and very limited
number of test-color samples..." (Color Science, Wyszecki & Stiles, 1982 2nd
ed.) Part of the problem with using CRI to judge any fluorescent source is
that the CRI "was designed originally to compare continuous spectrum sources
whose CRI's were above 90." (Lighting Application Bulletin, General
Electric) Since fluorescent sources are not continuous, it follows that some
other method should be used to rate their ability to simulate daylight.
ColorView achieves a CRI of 98.
Technical Difficulties in Producing Accurate Artificial Daylight
It is generally accepted that daylight created by appropriately filtering an
incandescent light source more accurately simulates real daylight than
"daylight" fluorescent sources. The reason is because of the narrow spikes
found in the SPD's of all fluorescent sources. The mercury discharge used in
fluorescent sources emits light at specific wavelengths resulting in narrow
spectral spikes. Naturally occurring daylight does not have similar spectral
spikes. These narrow spectral spikes can throw off your ability to
accurately discern color differences. Properly filtered incandescent light
does not have narrow spikes in its SPD and therefore more closely models
natural daylight.
Influence of the Automotive Industry
The American automobile industry has had a significant effect on the light
booth market. The Detroit Color Council (which works closely with the "big
three") recently released a new spec. for viewing booths. That document
indicates that filtered incandescent light shall be used to reproduce
daylight. Further, it states: "Daylight is the most important lighting
source to make a color determination..." and it establishes "6500 +/-200K" as
the correct daylight color temperature for your light booths. The DCC
emphasized that light booths be maintained as "precision instruments," and
they stressed that the daylight's color temperature is more critical than the
intensity. In addition, the DCC listed the following 10 reasons why the
daylight color temperature in a booth can change:
1. Color temperature of halogen lamps.
2. Light intensity of lamps.
3. Inspection light to perch dimensional relationship.
4. Color of inspection perch.
5. Condition, age, and color of daylight filters.
6. Cleanliness of inspection lights.
7. Age and condition of lamps.
8. Voltage of input power.
9. Age and condition of electronics.
10. Ambient viewing condition, presence of smoke, dust, bright clothing or
colors, ambient light entering the area.
Calibrating a Light Booth: Improves Quality Control
The rise of ISO 9000 standards and the demand by the automotive industry to
improve the overall quality of their products has brought forward the issue
of calibrating the light used in a light booth to NIST-traceable standards.
As logical as this may seem, it is revolutionary in the industry of light
booths because until recently, people have not realized the importance of
calibrating their light booths.
However, given all the reasons why the color temperature inside a booth can
change, and knowing that thousands of dollars in time, effort, and material
can depend on the judgment made in a booth, it is imperative that companies
demand that their light booths be maintained at the highest possible level.
Spectrophotometers and other instruments used in color analysis are
frequently checked and recalibrated. Similarly, the color temperature of
light booths should be checked frequently and recalibrated when necessary.
This simple step will improve a companies color quality control and reduce
the amount of wasted time, effort, and raw materials.
Tailored Lighting Inc. (TLI)
The three goals of TLI's light booth products are:
1) Provide the highest quality daylight by using a filtered incandescent source,
2) Provide a quick and easy way for the user to calibrate the color
temperature of the daylight in the booth to an officially recognized
(i.e. NIST-traceable) source,
3) Fulfill step #1 and #2 at a reasonable cost.
Products that provide calibrated Daylight
TLI offers two light booth products:
1) ColorView LightBooth 5000K & 6500K
The ColorView LightBooth is TLI's best selling lighting system. It has the
primary lighting conditions crucial for critical color evaluation in the
laboratory: 6500K or 5000K daylight, incandescent light, and all lighting
conditions in between. Its patented ColorView technology provides the
user with the ability to calibrate the color temperature of the booth.
No other light booth on the market has this ability. The unit measures
14" X 14" X 14", weighs approximately 10 lb., does not require any
special wiring, and is priced at $445 for the 5000K and $795 for the 6500K.
When price, performance and capabilities are all considered, the choice is
obvious: Tailored Lighting makes the best light booths products in the world.
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